1. Field of the Invention
The present invention relates to an organic light-emitting device. More particularly, the invention relates to an organic light-emitting device having a light-emitting element comprising a pair of electrodes and an organic light-emitting material layer sandwiched by the electrodes. This device is applicable to various types of display device.
2. Description of the Related Art
Electro-Luminescence (EL) elements are one of the types of light-emitting elements designed for display devices EL elements are divided into two types, xe2x80x9corganic EL elementsxe2x80x9d using an organic material layer as the light-emitting layer and xe2x80x9cinorganic EL elementsxe2x80x9d using an inorganic material layer as the light-emitting layer.
The basic configuration of organic EL elements comprises an anode, a cathode, and a thin, layer-shaped organic EL structure made of an organic light-emitting (i.e., electro-luminescent, EL) compound or compounds sandwiched by the anode and the cathode. If a proper voltage is applied across the anode and cathode, holes are injected from the anode into the organic EL structure and at the same time, electrons are injected from the cathode into the same structure, resulting in recombination of the holes and electrons. Due to the energy generated by the recombination, the molecules of the material that forms the EL layer are excited. These molecules thus excited will be returned to their energetic ground states, in other words, deactivated. During this deactivation process, a light-emitting (i.e., EL) phenomenon will occur. The above-described organic EL elements utilize this phenomenon.
The organic EL structure comprises an organic layer termed the xe2x80x9clight-emitting layerxe2x80x9d that emits light using the recombination of holes and electrons. If necessary, an organic layer termed the xe2x80x9chole-transportation layerxe2x80x9d and/or an organic layer termed the xe2x80x9celectron-transportation layerxe2x80x9d are additionally provided. The xe2x80x9chole-transportation layerxe2x80x9d has a property that injection of holes is easy while transportation of electrons is difficult. The xe2x80x9celectron-transportation layerxe2x80x9d has a property that injection of electron is easy while transportation of holes is difficult.
Thus, if the El structure comprises only the light-emitting layer, it has a single-layer structure. If the EL structure comprises at least one of the hole-transportation layer and the electron-transportation layer along with the light-emitting layer, it has a multi-layer (i.e., two- or three-layer) structure.
In recent years, organic EL elements have been vigorously researched and they are on the way to put them into practice. These prior-art organic EL elements have the basic configuration comprising a transparent electrode (i.e., a hole-injection electrode, anode), a thin hole-injection material layer formed on the transparent electrode, a light-emitting layer formed on the hole-injection material layer, and a metal electrode (i.e., an electron-injection electrode, cathode). The transparent electrode for hole injection (i.e., anode) is made of an indium sin oxide (ITO) or she like. The hole-injection material layer is made of triphenyl diamine (TPD) or the like, which is formed by evaporation. The light-emitting layer is made of a fluorescent material, such as an aluminum quinolinol complex (Alq3). The metal electrode for electron injection (i.e., cathode) is made of a metal having a low work function, such as AgMg.
These prior-art organic EL elements afford us very high brightness of several hundreds cd/m2 to several hundreds thousands cd/m2 as a voltage of approximately 10 V. Therefore, they have been drawing our attention because of their application fields, such as lighting, light sources, display devices for so-called OA (office automation) instruments, household electrical appliances, automobiles, motorcycles, and airplanes.
For example, the prior-art EL elements designed for these application fields have the configuration that the organic layers (e.g., the light-emitting or EL layer) are sandwiched by the scan electrodes (i.e., common line electrodes) serving as the electron-injection electrodes and the data electrodes (i.e., segment line electrodes) serving as the hole-injection (transparent) electrodes. The organic layers and the scan and data electrodes are located on a transparent substrate, such as a glass plate.
The display devices including the light-emitting elements are divided into two groups, the xe2x80x9cmatrix typexe2x80x9d and the xe2x80x9csegment typexe2x80x9d. With the xe2x80x9cmatrix typexe2x80x9d, the light-emitting elements, which are arranged in the form of a matrix, are driven by the scan and data electrodes to form a dot matrix on the display screen. Desired information, such as images and/or characters, is displayed on the screen as a set of the dots. On the other hand, with the xe2x80x9csegment typexe2x80x9d, a set of display segments is prepared in advance. These segments are separately formed in the screen, each of which has a predetermined shape and size. Desired information is displayed on the screen as a combination of the segments.
With the segment-type display devices, each of the display segments may be driven separately from each other. Unlike this, with the matrix-type display devices, the dynamic drive method has been adopted in such a way that the scan lines and the data lines are driven in the time-division manner.
The configuration of the light-emitting device comprising the light-emitting element is divided into two types, i.e., the xe2x80x9csubstrate-surface-emissionxe2x80x9d type and the layer-surface-emissionxe2x80x9d type.
With the xe2x80x9csubstrate-surface-emissionxe2x80x9d type, the four-layer structure made of the transparent substrate, the transparent electrode (i.e., the hole-injection electrode or anode), the light-emitting layer, and the metal electrode (i.e., the electron-injection electrode or cathode) formed in this order is used. The light generated in the light-emitting layer is emitted to the outside from the surface of the substrate by way of the transparent electrode and the transparent substrate. An example of this type is disclosed in the paper, Applied Physics Letters, Vol. 51, No. 12, Sep. 21, 1987, pp. 913-915.
On the other hand, with the xe2x80x9clayer-surface-emissionxe2x80x9d type, the four-layer structure made of the substrate, the metal electrode (i.e., the electron-injection electrode or cathode), the light-emitting layer, and the transparent electrode (i.e., the hole-injection electrode or anode) formed in this order is used. The light generated in the light-emitting layer is emitted to the outside from the opposite surface of the transparent electrode to the substrate by way of the transparent electrode. An example of this type is disclosed in the paper, Applied Physics Letters, Vol. 65, No. 21, Nov. 21, 1994, pp. 2636-2638.
A typical configuration of the prior-art organic light-emitting devices of this type is shown in FIG. 1.
The light-emitting device of FIG. 1 comprises a light-emitting element 100 formed on a transparent substrate 105. The substrate 105 is formed by a transparent plate. A current-supplying element 102 is connected to the element 100 to supply an electric current to the element 100. A switching element 101 is connected to the element 102.
Typically, although not shown here, the light-emitting element 100 has a five-layer structure made of a transparent electrode (i.e., a hole-injection electrode or anode), a hole-injection layer, a light-emitting layer, an electron-transportation layer, and a metal electrode (i.e., an electron-injection layer or cathode) stacked on the substrate 105 in this order. The transparent electrode is the nearest to the substrate 105. As each of the current-supplying element 102 and the switching element 101, a Thin-Film Transistor (TFT) of the Metal-Oxide-Semiconductor (MOS) type is typically used.
With the prior-art light-emitting device of FIG. 1, the light emitted from the light-emitting layer of the light-emitting element 100 travels vertically through the substrate 105 to its surface 106. Since the refractive index of the substrate 105 is greater than that of the air, the component of the light whose incident angle is equal to the critical angle or greater is unable to go to the outside of the substrate 105. This means that this component will be unavailable light 104 due to total internal reflection. On the other hand, the component of the light whose incident angle is less than the critical angle is able to go to the outside of the substrate 105. This means that this component will be available light 103.
For this reason, the prior-art device of FIG. 1 has a high proportion of loss of light and thus, the obtainable ratio of the output light to the input power is as low as approximately 20%. As a result, if proper brightness is ensured as desired, there arises a problem that the power consumption increases and at the same time, the device lifetime decreases. Conversely, if power consumption is lowered, there arises another problem that desired brightness is not obtainable.
The above explanation relates to the prior-art organic light-emitting device of the substrate-surface-emission type. With the prior-art organic light-emitting device of the layer-surface-emission type, a similar problem will occur, because a silicon oxide layer or a silicon nitride layer is usually used as the surface protection layer.
Similar techniques to the invention are disclosed in the Japanese Patent No. 2692671 issued on Sep. 5, 1997, the Japanese Non-Examined Patent Publication No. 10-183243 published on Jul. 21, 1998, and the Japanese Non-Examined Patent Publication No. 11-214162 published on Aug. 6, 1999.
The Japanese Patent No. 2692671 discloses the structure of a resonator-type organic thin-film EL element. In this structure, a depression is formed in the transparent substrate. The light-emitting section is located in the depression of the substrate. However, with this structure, a desired directionality is given to the light to be emitted. Therefore, this structure is quite different from the structure of the invention.
The Japanese Non-Examined Patent Publication No. 10-189243 discloses the structure of an EL display device. This device comprises a lower electrode, a lower insulating layer, a light-emitting layer, an upper insulating layer, and an upper electrode stacked on a main surface of a substrate. Tapered depressions are formed on the main surface of the substrate. The interface between the lower electrode and the lower insulating layer, the interface between the lower insulating layer and the light-emitting layer, the interface between the light-emitting layer and the upper insulating layer, and the interface between the upper insulating layer and the an upper electrode are formed to be parallel to the tapered surface of the depression.
With the structure of the Japanese Non-Examined Patent Publication No. 10-19243, however, the depressions need to be formed in advance and thus, there is a difficulty in the fabrication processes. Moreover, since the lower insulating layer is formed between the lower electrode and the light-emitting layer and at the same lime, the upper insulating layer is formed between the upper electrode and the light-emitting layer, there is a problem that the injection efficiencies of electrons and holes into the light-emitting layer degrade, and that the fabrication cost rises.
The Japanese Non-Examined Patent Publication No. 11-214162 discloses the structure of an organic EL element, which comprises at least one thin organic layer located between a pair of opposing electrodes. Minute protrusions are formed on one of the electrodes located at the emission side of the element while minute depressions are formed on the other electrode corresponding to the respective protrusions.
With the structure of the Japanese Non-Examined Patent Publication No. 11-214162, however, no explanation about the inclination angle of the minute protrusions is disclosed. Moreover, the light generated in the organic layer is taken out of the element by way of at least one of the electrodes. Therefore, there is the need to form the electrode or electrodes through which the light is taken out with a material having high transmittance of light. This means that there is a disadvantage that the type of applicable materials for the electrodes is limited.
The invention was created to solve the problems or disadvantages of the above-described prior-art structures and to raise the output efficiency of light more.
Accordingly, an object of the present invention is to provide an organic light-emitting device that increases the proportion of available light as much as possible.
Another object of the present invention is to provide an organic light-emitting device that produces higher brightness with less input power.
Still another object of the present invention is to provide an organic light-emitting device that prevents the total internal reflection effectively at the interface between the device and the air, thereby raising the light-outputting efficiency.
A further object of the present invention is to provide an organic light-emitting device that suppresses its inner absorption of light to thereby improve the utilization efficiency of light.
A still further object of the present invention is to provide an organic light-emitting device that suppresses unavailable emission of light to thereby improve the light-emitting efficiency of the light-emitting element.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the invention, an organic light-emitting device is provided. This device comprises:
(a) a base having a surface;
(b) a first electrode and a second electrode formed on the surface of the base;
the first and second electrodes being located to be opposite to each other;
(c) a light-emitting material layer located between the first and second electrodes;
the light-emitting material layer having a first part and a second part;
the first part being inclined with respect to the surface of the base at an inclination angle within a range from 45xc2x0 to 90xc2x0;
the second part being approximately parallel to the surface of the base; and
(d) a carrier-blocking layer formed between the first electrode and the second part of the light-emitting material layer;
the carrier-blocking layer having a function of blocking carriers emitted from the first electrode into the light-emitting material layer;
wherein the first part of the light-emitting material layer forms as a light-emitting region while the second part of the light-emitting material layer forms a non-light-emitting region due to the function of the carrier-blocking layer;
and wherein output light is emitted from the light-emitting region of the light-emitting material layer.
With the organic light-emitting device according to the first aspect of the invention, the carrier-blocking layer is formed between the first electrode and the second part of the light-emitting material layer, where the second part is approximately parallel to the surface of the base. The first part of the light-emitting material layer, which is inclined to the surface of the base at the inclination angle within the range from 45xc2x0 to 90xc2x0, serves as the light-emitting region and at the same time, the second part of the light-emitting material layer serves as the non-light-emitting region.
Thus, the output light is emitted from only the light-emitting region of the light-emitting material layer. This means that the output light penetrates the interface between the base and the outside air (or, the interface between the second electrode and the outside air) at smaller incident angles than the prior-art device.
Therefore, if the output light is emitted to the outside by way of the base, internal total reflection in the base is effectively suppressed. If the output light is emitted to the outside by way of the second electrode, internal total reflection in the second electrode is effectively suppressed.
As a result, unavailable light is effectively decreased and thus, the proportion of available light is increased as much as possible. In other words, unavailable emission of light is suppressed to thereby improve the light-outputting efficiency.
Furthermore, due to the raised light-outputting efficiency, higher brightness can be produced with less input power.
According to a second aspect of the invention, another organic light-emitting device is provided. This device comprises:
(a) a base having a surface;
(b) a first electrode and a second electrode formed on the surface of the base;
the first and second electrodes being located approximately to be opposite to each other;
(c) a light-emitting material layer located between the first and second electrodes;
the light-emitting material layer having a first part inclined with respect to the surface of the base at an inclination angle within a range from 45xc2x0 to 90xc2x0; and
(d) a carrier-blocking layer formed between the first electrode and the second electrode;
the carrier-blocking layer having a function of blocking carriers emitted from the first electrode into the light-emitting material layer;
wherein the first part of the light-emitting material layer forms a light-emitting region while the carrier-blocking layer forms a non-light-emitting region;
and wherein output light is emitted from the light-emitting region of the light-emitting material layer.
With the organic light-emitting device according to the second aspect of the invention, the first part of the light-emitting material layer forms a light-emitting region while the carrier-blocking layer forms a non-light-emitting region. Thus, there are the same advantages as those of the device according to the first aspect.
According to a third aspect of the invention, a still another organic light-emitting device is provided. This device comprises:
(a) a base having a surface;
(b) a first electrode formed on the surface of the base;
the first electrode having an approximately rectangular or trapezoidal cross section perpendicular to the surface of the base;
the first electrode being elongated to extend along the surface of the base;
the first electrode having a top face opposite to the surface of the base and a pair of side faces located on opposite sides of the first electrode;
(c) a carrier-blocking layer formed to contact the top face of the first electrode;
the carrier-blocking layer having a function of blocking carriers emitted from the first electrode;
(d) a light-emitting material layer formed to extend along the first electrode in such a way as to contact the pair of side faces of the first electrode and the carrier-blocking layer; and
(e) a second electrode formed on the light-emitting layer;
wherein a first part of the light-emitting material layer that contacts the carrier-blocking layer forms as a non-light-emitting region while a second part of the light-emitting material layer that contacts the light-emitting material layer forms a light-emitting region;
and wherein output light is emitted from the light-emitting region of the light-emitting material layer.
With the organic light-emitting device according to the third aspect of the invention, because of substantially the same reason as the device of the first aspect, there are the same advantages as those of the device according to the first aspect are obtainable.
According to a fourth aspect of the invention, a further organic light-emitting device is provided. This device comprises:
(a) a base having a surface;
(b) a first electrode formed on the surface of the base;
the first electrode having an approximately rectangular or trapezoidal cross section perpendicular to the surface of the base;
the first electrode being elongated to extend along the surface of the base;
the first electrode having a top face opposite to the surface of the base and a pair of side faces located on opposite sides of the first electrode;
(c) a carrier-blocking layer formed to contact the top face of the first electrode;
the carrier-blocking layer having a function of blocking carriers emitted from the first electrode;
(d) a light-emitting material layer formed to extend along the first electrode in such a way as to contact the pair of side faces of the first electrode and the carrier-blocking layer; and
(e) a second electrode formed on the light-emitting material layer;
wherein a first part of the light-emitting material layer that contacts the carrier-blocking layer forms as a non-light-emitting region while a second part of the light-emitting material layer that contacts the light-emitting material layer forms a light-emitting region;
and wherein output light is emitted from the light-emitting region of the light-emitting material layer.
With the organic light-emitting device according to the fourth aspect of the invention, because of substantially the same reason as the device of the second aspect, there are the same advantages as those of the device according to the first aspect are obtainable.